a. Field of the Invention
Broadly speaking, this invention relates to waveguides. More particularly, in a preferred embodiment, this invention relates to optical waveguides in which mode mixing is deliberately encouraged with virtually no accompanying losses to radiation.
b. Discussion of the Prior Art
In recent years, considerable attention has been given to optical fibers which have parabolic or graded index profiles. Examples of such fibers are the fibers sold under the tradename "SELFOC", and the graded index fibers disclosed in U.S. Pat. No. 3,823,997, which issued on July 16, 1974 to D. C. Gloge and E. A. J. Marcatili. Graded index fibers, such as above-described, are superior in performance to more conventional, step-index fibers because all of the modes which propagate in the guide have very nearly the same group velocity; thus pulse spreading is minimized.
With respect to step-index fibers, one way to minimize the effect of pulse spreading is to deliberately encourage coupling among the various modes propagating in the guide; for example, by modulating the physical dimensions of the fiber, as taught in U.S. Pat. Nos. 3,666,348 and 3,687,514 which respectively issued on Mar. 30, 1972 and Aug. 9, 1972 to E. A. J. Marcatili, and S. E. Miller and S. D. Personick, or by modulating the refractive index of the fiber core as taught, for example, in U.S. Pat. No. 3,909,110 which issued to D. Marcuse on Sept. 30, 1975.
At first blush, it would appear that by combining both of the above techniques, that is to say, by deliberately encouraging mode coupling in a graded index fiber, one could obtain a fiber which is superior to any fiber heretofore available. Unfortunately, this did not prove to be the case in practice, because the differences between the propagation constants of neighboring modes in a graded index fiber are almost independent of the mode label, which made it very difficult to discriminate between coupling among guided modes and coupling from guided to radiation modes.
The problem, then, is to find a technique for enhancing mode conversion in a graded index fiber which does not at the same time result in increased radiation losses.